Two treatments/technologies have been described which are aimed at improving productivity in respect of plants grown in nitrogen-deficient soil: (a) the fertilization with nitrogenous compounds (e.g., urea) and (b) the inoculation with nitrogen-fixing bacteria. There exist two types of nitrogen-fixing bacteria: (b1) symbiotic bacteria which fix nitrogen in association with leguminous plants (e.g., Rhizobium sp. and Sinorhizobium sp.) and (b2) free-living nitrogen-fixing bacteria (e.g., Azospirillum brasilense and Azotobacter vinelandii). The first type was successfully employed to improve productivity in respect of crops grown in soils poor in nitrogen, but it is only applicable to leguminous plants. The second type could be employed with all kinds of plants (not only in respect of leguminous ones). However, it has not yet been stated that the nitrogen fixed by these bacteria is sufficient to overcome the nitrogen deficiency suffered by plants grown in soils poor in nitrogen. Despite this, species such as A. brasilense and A. vinelandii are currently employed in the formulation of inoculants since such bacteria produce phytohormones capable of stimulating root development. This is a feature of agronomic importance, but it bears no relation to the nitrogen-fixation ability of these bacteria.
The patent publication EP0108508 describes Escherichia coli strains modified with genes related to nitrogen fixation. However, the E. coli strains are not employed to improve productivity in respect of plants grown in soils poor in nitrogen, but in order to transfer the nitrogen-fixation ability to other microorganisms, such as Rhizobium japonicum. 